The circuit illustrated below is used as an electronic letter box. When letter is put inside, a LED glows which indicates the presence.

This circuit uses a popular timer I.C which is 555. I.C 555 is connected as comparator with pin 6 connected with positive supply, the output goes high-1 when the trigger pin 2 is at lower than 1/3 level of the supply voltage. Conversely the output goes low-0 , when it is above 1/3 level. So small change in the voltage of pin 2 is enough to change the output of pin 3 from 1 to 0 and 0 to 1. The output has only two states high and low and can not remain in any intermediate stage. It is power by 9V battery for portable use. The circuit is economic in power consumption. Pin 4,6&8 is connected to the positive supply and pin 1 is grounded.

To detect the present of letter we have used LDR and a source of light. LDR is a special type of resistance whose value depends on the brightness of the light which is falling on it. It has a resistance of about 1 megaohms when in total darkness, but a resistance of only about 2-5 k ohms when brightly illuminated. It responds to a large part of the light spectrum.
The source of light and LDR is so adjusted in the letter box that light will directly fall on the LDR but when letter is kept inside then it will block the beam of light and LDR will be under darkness.
We have made a potential divider circuit with LDR and 100 K variable resistance
connected in series. Voltage is directly proportional to conductance so more voltage we will get by this divider when LDR is getting light and low voltage in darkness. Divided voltage is given to pin 2nd of 555. As soon as LDR gets dark the voltage of the pin 2 drops 1/3 of the supply voltage and pin 3 gets high and LED glows.

We can also use two LED at output pin 3, for present LED 1 and for absent LED 2. For this method one LED is connected as forward bias and other is connected as reverse bias to indicate both high and low conditions. To limit the current of LED resistance is used in series.
For source light, use Red or White LED. For LED 2 and 3 you can use any other color- green, red, yellow or blue.